Using the UE4 mannequin for scale in Blender
In Blender you can input math operations instead of values. In your case select the mannequin followed by S 183/210 This will reduce the size to that you had in UE4.
Putting Scale X,Y and Z transform fields to 183/210 and hitting Enter works as well. Also, this is the way to use with i.a. modifiers and every field.